Issue 40

L. Zou et alii, Frattura ed Integrità Strutturale, 40 (2017) 137-148; DOI: 10.3221/IGF-ESIS.40.12 139 Results show that there exists a transition point in the fatigue crack growth curve in the near-threshold regime, and the stress intensity range of the fatigue threshold decreases with the increasing of stress ratio. Currently, there is still a lack of an objective and comprehensive evaluation of the great many factors which influence the fatigue life of the welded structure. To establish the mathematical model of different influence factors, neighborhood rough set theory is used to find the core factors which influence the fatigue life of the aluminum alloy welded joints based on the data itself rather than on any other prior knowledge. Fatigue characteristics domains are then determined according to the key influence factors and the S-N curves are fitted in each domain subsequently. M ETHODOLOGY Basic Principle of the Nodal Force Based Structural Stress he normal structural stress at each node from elementary structural mechanics theory is given by s m b      (1) / y m f t   (2) 2 6 / x b m t   (3) where / / y y x x f F l m M l  , = = is the line force and moment in the weld tow shown as Fig. 1, F y is the nodal force, M x is the moment around the weld toe. Figure 1 : Definition of linear force. Fracture mechanics is employed to estimate the fatigue life of welded joints. The stress intensity factor in crack propagation theory can be calculated as [2]: * [ ( / ) ( / )], m m b b K t f a t f a t      (4) where a is the crack depth, t * is a ratio of actual thickness t to a unit thickness. ( / ) m f a t and ( / ) b f a t are membrane stress and bending stress as a function of crack growth degree respectively. According to the Paris crack growth law, the prediction of the life cycle from an infinitesimally small crack to final failure can be expressed as: / 1 1 2 / 0 * * ( / ) 1 ( ) ( ), ( ) ( ) m a t m s n m a t kn t d a t N t I r C M K C          (5) where M kn = K / K n is the notch stress magnification , K represents the total K due to both the far-field stress and the local notch stress effects and K n represents only the far-stress contribution to the stress intensity factor. I(r) is a dimensionless function of r and m is the crack growth exponent, which is set to be 3.6 in ASME [13]. A Master S-N curve can be established according to Eq. 6 based on a set of welding fatigue data. The Eq. SS can then be expressed as: 1 2 * ( ) ( ) m m s t I r       (6) T f (x) f y2 F y2 x l element f y1 F y1 y